1. Field of the Invention.
The invention relates to an x-ray mammography apparatus.
2. Technological Background
Breast cancer is a major cause of death and disability among women in most western countries. Its incidence increases with age. At age 40, the incidence of invasive breast carcinoma is about 80 cases per 100,000 women. At the age of 50, the rate increases to 180 cases per 100,000 women. At the age of 60, the rate is about 240 cases per year per 100,000 women. In all, one out of eleven women in the United States develop breast cancer in their lifetime, the vast majority of cases occurring in the middle and later years of life. Accordingly, annual mammography examination becomes a part of many women's lives above the age of 35 to 40.
Mammography is x-ray examination of a woman's breasts to detect the presence of potentially cancerous tumors. Dr. Ingvar Andersson, writing in the bulletin Medical Radiography and Photography, volume 62, number 2, 1986, at page 4, observed that "few radiographic examinations are as sensitive to improper technique as mammography." Mammography involves imaging structures that have small differences in x-ray radiation absorption characteristics. Successful mammography requires dedicated x-ray apparatus be used for examination. Special, highly monochromatic x-ray sources and good beam filtration are required. Breast compression is is used to reduce scatter by decreasing the thickness of the breast and to separate the layers of tissue. Meticulous film processing is done to obtain the maximum possible image contrast. Nevertheless, mammography requires large radiation dosages when compared to conventional chest x-rays. A conventional posterior/anterior (P/A) chest x-ray examination results in a radiation dosage of 30 millirads, a mammogram examination for a woman with breasts of average size and density involves a dosage of about 1000 millirads, a factor of increase of over 30.
Mammography presently offers the most sensitive technique for early diagnosis of cancerous growths, including the potential for demonstrating non-palpable cancer and precancerous growths. Early detection, when the tumor is small, offers the best prognosis for the cancer patient.
However, the potential adverse effects of x-ray mammograpy, particularly the relationship of annual x-ray examination of women's breasts with possible induced cancers in breast tissue, poses a dilemma to treating physicians. An increased incidence of breast cancer has been observed among women subjected to ionizing radiation below the age of 30 at the time of exposure. Epidemiologic data relating to the carcinogenic effect of exposing breasts to ionizing radiation suggests, at high dose levels, a linear relationship of increasing cancer risk per rad per year of radiation received. While linear extrapolation of such data to dosage rates commonly associated with mammography (typically on the order 1000 millirads per examination with state of the art equipment) is not necessarily valid, the increased incidence rates suggest that a number of cancers per year can be anticipated from this examination process. Physicians, thus, are compelled to balance the risks of missing detection of an early cancer with the risk of inducing cancer in an otherwise healthy person when they recommend a schedule of mammography examinations.
The critical parameters relating radiation dosages to cancer incidence rates suggested by the epidemiological data are, (1) dosage per examination, and (2) cumulative radiation received by the patient. Reduction of radiation exposure per examination promises a number of benefits. Among these would be the ability to maintain a schedule of examinations reasonably calculated to detect early cancers with a reduced risk of induced cancer from each examination and a reduced risk of induced cancer from the cumulative dosage over the lifetime of a woman.
The low difference in x-ray absorption characteristics between cancerous and noncancerous tissue has made mammography particularly susceptible to imaging problems caused by scattered radiation. Scattered radiation is secondary radiation produced in the radiated breast, and emanating in all directions. According to some studies, scattered radiation may amount to up to 80% of the total radiation exposing the film in a mammogram, resulting in image deterioration and fogging by loss of contrast. This means that imaging of the areas of the breast is done with only about 20% of the radiation exposing the film. Improvement of the ratio can be effected using techniques for suppression of scattered radiation.
Bucky grids are positioned between an object being x-rayed and film being exposed to the x-rays to prevent secondary radiation from reaching the film and thereby improve image contrast on the film. They also reduce primary radiation reaching the film, which in turn may require a compensating increase in radiation exposure of the breast. A Bucky grid typically includes thin strips of x-ray radiation absorbing material, called lamellae, substantially aligned with the incident course of the radiation from the x-ray source, with x-rays transmitted through the gaps between the lamellae. Only a portion of the x-ray radiation substantially aligned with radiation emitted from the x-ray source is transmitted through the gaps in the grid. The degree of alignment required is a function of the ratio of the height of the lamellae to the width of the gaps between lamellae. Radiation not aligned with radiation from the source is blocked by the grid from reaching the film. The proportion of aligned x-ray radiation transmitted through the grid is a function of the ratio of the thickness of the lamellae to the width of the gaps between the lamellae, the absorption characteristics of the interspace material and, in current Bucky grids, the precision of the focusing of the grid.
Bucky grids used in mammography have come as either stationary flat grids, or reciprocating flat grids. A stationary Bucky grid will cast a shadow where the image is formed, unless the grid is composed of extremely fine lamellae (on the order of 80 lines per centimeter). A grid which is reciprocated tangentially through an x-ray beam during exposure blurs the shadow cast by the grid. It is a rule of thumb that a minimum of 15 to 20 lamellae should pass each exposed point of a film to effectively blur the shadow of the lamellea regardless of the duration of the exposure.
The employment of bucky grids incorporating extremely fine lamellae and short grid travels has been driven in part by the requirements of imaging as much of the juxtathoracic portion of the breast as possible. Two favored views used to insude reasonably complete imaging of the breast are the so called "oblique" and "lateral" views. A compact bucky grid aids in positioning the woman undergoing examination. Flat grids with tightly packed lamellae traveling over a short distance have been employed for ease of positioning against the breast, particularly for oblique views of the breast. Thus designers of both stationary and reciprocating grids have repeatedly reduced lamellae width.
Stationary grids used in mammography are extremely shallow (about 1 mm in height) and have densely packed lamellae. The height to width ratio of the gap between lamellae is usually about 3.5:1. Reciprocating grids generally have a height to width gap ratio of 5:1. Dedicated mammography machines of either type have used a flat grid with reinforcing spacing material between lamellae. Focus of the grid is the extent to which the lamellae are aligned with radiation from the source. If the grid is a reciprocating grid, maximum focus occurs only at the center point of the grid's travel. Thus, only at the center point the grid's travel is the theoretical maximum amount of direct radiation passing through the grid. As the grid becomes noncentered, the lamellae move off center and progressively larger proportions of useful radiation are absorbed.
Mammography machines, employing reciprocating, flat Bucky grids, have focused on reducing the thickness of the lamellae and the width of the interstitial gaps to a minimum to provide minimum offset of the grid during operation. The relationship between the gaps and the shadows cast by the lamellae is called the open area. In an ideal Bucky grid, it may be 60% at 28 kv in the center position. Because the grids move off center during operation and due to mechanical imperfections in the mammography grids, the effective open area can be reduced to as low as 37% at the same energy setting in the maximum off center position. By increasing the number of lamellae per given unit of length, the degree to which the noncentered lamellae are out of alignment with respect to the x-ray source is reduced. In stationary grids, lamellae have been made continuously thinner and more closely packed to eliminate the shadow problem. However, for any given material, such as lead, tantalum, tungsten, or any other heavy metal, there is a minimum thickness below which the material becomes inefficient at absorbing x-ray radiation. Thus, as the number of lamellae per unit length is increased, the total area of lead absorbing material presented to the x-ray source as a proportion of the area of the grid increases, reducing direct transmission of x-ray radiation and thereby requiring an increase in exposure to the breast.
Bucky grids with high ratios of lamellae height to gap width reduce transmission of scatter and secondary radiation through the Bucky grid. However, the focus problem of flat grids in mammography limits the lamellae height to gap width ratio to about 5:1, and in some cases has forced utilization of ratios as low as 2:1. These low ratios result in excessive scatter radiation being passed by the Bucky grids, lowering the quality of the image. Mammography grids, having extremely thin lamellae requiring support to retain shape are inefficient in x-ray primary radiation transmission.